1. Field of the Invention
The present invention relates to the rotogravure printing of an uncoated paper web with printing inks which contain water-immiscible solvents.
2. Description of the Prior Art
Uncoated, highly supercalendered and highly filled papers are produced on a large scale and are used as the print carrier in magazine and illustration rotogravure. These papers are referred to hereinafter as natural rotogravure papers. They are printed with printing inks, which contain a considerable proportion of water-immiscible solvents. Such solvents are, for example, toluene, xylene and benzene.
Constantly increasing quality of the natural rotogravure paper is being demanded because of changes in the paper making and rotogravure processes. Specifically, this is due to the increasing speed of the paper machines and the consequently accelerated dewatering on the Fourdrinier, since with twin wires, a less homogeneous paper sheet would be produced as well as the increasing speeds of the rotogravure printing machines.
One of the major problems resulting from the higher machine speeds is that a lower printing ink viscosity is required which, in turn, affects the "migration" of the printing ink into the paper.
The migratory properties of a printing ink are regarded as good if the ink, in the brief period between application and drying, does not migrate away from the point of application so that the contours of the ink on the printed and dried paper are the same as they were when the ink was applied to the paper, i.e., the image is sharp. In the case of poor migration properties, the printing ink penetrates into the paper and spreads out, which leads to a nonuniform and blurred printing image. In black areas, for example, insufficient blackening occurs and the printed image has inferior gloss. At the same time, the uneven distribution of fibers and filler material in the microregion of the surface can be observed in the printed image.
Various attempts have been made to improve the printability of papers. German Pat. No. 828,478 proposes that various minerals, such as, zeolite, be added to the fibrous material or that these minerals, in combination with starch or different binders be applied to the surface of the paper in a preparation step. At the same time, the penetration of oily molecules or of other printing fluids is promoted by the channels which traverse the interior of these minerals.
German Pat. No. 844,402 discloses the addition and distribution of discrete clay particles as a filler in the structure of the paper to prevent a running of the printing ink because of their adsorption effect. The use of oil-absorbing substances for improving the printing properties is also described in British Pat. No. 1,093,041. These substances are synthetically produced pigments having an amorphous structure and are used as fillers in a conventional manner.
The use of extruded minerals, such as, kaolin or attapulgite has been suggested in U.S. Pat. No. 3,433,704 for the production of newsprint. The oils used in newsprint ink tend to migrate through the paper and give rise to the formation of translucent areas in the printed paper. The use of the extruded minerals is intended to prevent the printed image which is applied to one side of the paper from showing through on the side by limiting the reduction in opacity caused by the oils.
These proposals are based on utilizing the adsorption properties of the different minerals for printing inks or on increasing the printing opacity. This approach, which is also adhered to in the reference "Physical Chemistry of Pigments in Paper Coating", page 422, has not been practiced in rotogravure printing with solvent-containing printing inks, i.e, printing inks containing toluene. This may primarily be attributed to the fact that the construction of rotogravure inks is completely different from that of newsprint inks. The latter having a significantly higher viscosity of about 50 Pascal seconds, while rotogravure inks have an average viscosity of 10 and a maximum viscosity of 20 Pascal seconds. In actual practice, however, viscosities of 4 Pascal seconds are also used in rotogravure printing. The already mentioned oils, predominantly mineral oils, are used as color carriers in printing newsprint, while more volatile solvents, especially toluene and benzene, in which natural or synthetic resins are dissolved, are used in rotogravure printing. The color carriers of newsprint inks remain in the paper while the toluene used as a solvent for the resins, evaporates immediately.
However, newsprint paper also has a generally different construction than the natural rotogravure papers addressed in the present application. Specifically, natural rotogravure papers have the maximum possible amount of fillers added. Additionally, they have a higher chemical pulp content and differ in their physical properties, e.g., they have a much higher density and higher smoothness which is obtained by a supercalandering process.
Newsprint paper, on the other hand, is only machine-calandered, is run with the addition of only insignificant amounts of filler and has a density of about 0.6 g/cc.
The enveloping of fibrous materials with hydratable colloidal, film-forming clays is disclosed in German Pat. Nos. 2,451,216 and 2,608,239. German Pat. No. 2,451,216 deals with a acceptor paper, in which hydratable, colloidal clays or fibrous materials coated with such clays are contained as color acceptors for suitable color precursors. On the other hand, German Pat. No. 2,608,239 describes an image receiving material for electrophotographic processes, in which hydratable, film-forming, colloidal clays become effective to fix the water-extractable harmful substances of the type released by the thermofixation of toner particles.
These patents disclose the use of such enveloped fibers which have a high adsorptive power for that particular function.